1. Field of the Invention
The present invention relates to a firing installation. It also relates to a method for operating such an installation.
2. Discussion of Background
In accordance with the pure air regulations, it is necessary to ensure--when operating a firing installation --that the pollutants occurring as a result of tile combustion do not exceed the maximum permitted emission figures. This applies not only during correct operation under load but also during the whole of the starting phase. For this purpose, it has become known to mix a proportion of combustion gases with the fresh air, i.e. to operate combustion gas recirculation. In order to ensure that ignition of the mixture occurs, particularly during the starting phase, it is necessary to keep the recirculation rate as small as possible. The recirculation rate can be appropriately increased later. The recirculation rate necessary can be set by appropriate measures. A low combustion gas recirculation rate, however, never produces the minimum possible pollutant emission figures. Faulty admixture of the recirculating gas and fresh air is also detrimental to pollutant emissions. On the other hand, the problem cannot be avoided by deliberately operating with a higher recirculation rate because, during transient operation, this would inevitably introduce the danger that ignition and flame stability could be impaired.
A firing installation known from U.S. Pat. No. 5,147,200 to Knopfel et al. includes a fan which acts outside the envelope of the firing installation and which induces fresh air and mixes it with a certain proportion of combustion gases induced from the combustion space. The resulting fresh air/combustion gas mixture passes through a first heat exchanger on its way to the combustion space, the thermal preparation of this heat exchanger being provided by the combustion gases supplied. In the combustion space itself, this mixture flows through a heat exchanger which is positioned there. Before this mixture is supplied to a burner as combustion air, it experiences a further admixture of combustion gases by means of a main jet injector. This combustion gas admixture technique assumes a relatively long further mixture-formation distance upstream of the inlet into the internal space of the burner. In addition, inducing combustion gases centrally, as is done in this method, can always lead to form instability.